Radio signals in wireless communication systems are subject to frequency and time selective fading. These problems are caused by multipath propagation and Doppler shifts in wireless channels. Orthogonal Frequency Division Multiplexing (OFDM) is one technique for transmitting signals (symbols) at high bit rates while minimizing these problems, see Cimini, “Analysis and simulation of a digital mobile channel using orthogonal frequency division multiplexing” IEEE Trans. on Comm., COM-33, pp. 665–675, July 1985.
OFDM systems split the symbols over parallel low bit-rate sub-channels using frequency multiplexing with a minimum frequency spacing to achieve orthogonality. OFDM is inherently robust against frequency selective fading because each of the narrow band sub-channels occupies only a small portion of the total spectrum where the frequency response of the sub-channel is, for practical purposes, locally flat.
The robustness against multipath interference also comes from the presence of a guard interval separating adjacent OFDM symbols. If the channel delay spread is less than the guard interval, then intersymbol interference (ISI) will not affect the actual OFDM symbol. The guard interval can be discarded at the receiver.
Channel estimation has a substantial influence on the overall performance of the system. In the absence of channel information, differential detection is usually used at the expense of a 3 to 4 dB loss in signal to noise ratio (SNR) compared to coherent detection, see Li et al. “Robust channel estimation for OFDM systems with rapid dispersive fading channels,” IEEE Trans. on Comm., vol. 46, pp. 902–915, July 1998.
To make coherent detection possible, an efficient channel estimation process is necessary. Insertion of pilot signals in OFDM provides a base for reliable channel estimation. One class of pilot assisted estimation processes uses fixed parameter linear interpolation, see Said et al. “Linear two dimensional pilot assisted channel estimation for OFDM systems,” 6th IEEE Conf. on Telecommunications, pp. 32–36, 1998, and Moon et al. “Performance of channel estimation methods for OFDM systems in a multipath fading channels,” IEEE Trans. on Consumer Electronics, vol. 46, No. 1, pp. 161–170, February 2000. These processes are very simple in implementation, however, large estimation errors are inevitable in case of a mismatch.
Optimal and sub-optimal linear one-dimensional (1-D), double 1-D, and two-dimensional (2-D) estimators in the minimum mean-squared error (MMSE) sense have also been described for pilot assisted channel estimation in terrestrial audio and television broadcasting, and fixed and mobile wireless communications, see Edfors et al. “OFDM channel estimation by singular value decomposition,” IEEE Trans. on Comm., vol. 46, pp. 931–939, July 1998, and Hoeher et al., “Two-dimensional pilot-symbol-aided channel estimation by Wiener filtering,” Proc. of IEEE Int. Conf. on Acoustics, Speech, and Signal Processing, ICASSP-97, vol. 3, pp. 1845–1848, 1997.
However, filtering requires the knowledge of the channel, such as the correlation function of the channel impulse response, which is usually unknown in wireless systems. Robust pilot assisted estimation schemes where channel statistics are matched to particular cases, are described by Edfors et al. in “OFDM Channel Estimation by Singular Value Decomposition,” IEEE Trans. on Comm., vol. 46, pp. 931–939, July 1998, and Li et al. in “Pilot-symbol-aided channel estimation for OFDM in wireless systems,” IEEE Trans. on Veh. Technol., vol. 49, No. 4, pp. 1207–1215, July 2000. However, there, robustness is obtained at the expense of performance loss.
U.S. Pat. No. 5,912,876 “Method and apparatus for channel estimation” to H'mimy describes a method for channel response estimation over a fast fading channel. A coded orthogonal frequency division modulated (OFDM) signal that includes main and pilot signal portions is generated. The coded signal is transmitted over the fading channel to a receiving unit where the main signal is detected, and an estimation of the frequency response of the fading channel is made using the coded pilot signals. The detected main signal and the estimated channel frequency response are used to estimate the main signal. The determination can be based on a channel inversion of the frequency response or new channel estimation combined with maximum likelihood sub-sequence estimation. The maximum likelihood sub-sequence estimation in H'mimy is used for choosing the most likely transmitted data sequence.
The H'mimy method requires that the transmitter codes both the data and the pilot signals. The method requires that the pilot signals are all ones. The estimation is based on the frequency response of the channel using only the pilot signals.
Therefore, there is a need for an improved method and system for channel estimation and signal detection for OFDM systems that do not necessarily rely on particularly coded pilot signals.